MyAssemblyWindow.cs

using UnityEngine;
using UnityEditor;
using UnityEngine.Timeline;
using UnityEngine.Playables;
using System.Collections.Generic;

public class MyAssemblyWindow : EditorWindow
{
    [MenuItem("Window/Alan Tools/Assembly")]
    public static void ShowWindow()
    {
        // Find or create window so it appears on the screen.
        EditorWindow.GetWindow<MyAssemblyWindow>("Assembly");
    }

    void OnGUI()
    {
        AddCategory("Characters");
        AddCategory("Props");
        AddCategory("Lighting");
    }

    private void AddCategory(string name)
    {
        var assets = AssetDatabase.FindAssets("t:Prefab", new[] { "Assets/_PROJECT/Assembly Prefabs/" + name });
        if (assets.Length == 0)
        {
            return;
        }

        var buttonStyle = EditorStyles.miniButton;

        GUILayout.Label(name, EditorStyles.boldLabel);

        var inHoriz = false;
        int i = 0;

        foreach (var guid in assets)
        {
            var path = AssetDatabase.GUIDToAssetPath(guid);

            // Get a clean name for the button (removing "Prefab Variant" style endings).
            var prefabName = path.Substring(path.LastIndexOf('/') + 1);
            prefabName = prefabName.Remove(prefabName.LastIndexOf('.'));
            if (prefabName.EndsWith(" Variant"))
            {
                prefabName = prefabName.Remove(prefabName.LastIndexOf(' '));
            }
            if (prefabName.EndsWith(" Prefab"))
            {
                prefabName = prefabName.Remove(prefabName.LastIndexOf(' '));
            }

            // Putting 4 per line because cannot work out how to flow them nicely to pack as many as will fit per line.
            // TODO: try https://docs.unity3d.com/ScriptReference/EditorGUIUtility.GetFlowLayoutedRects.html
            if (!inHoriz)
            {
                GUILayout.BeginHorizontal("box");
                inHoriz = true;
            }
            if (GUILayout.Button(prefabName, buttonStyle))
            {
                AddPrefabInstance(prefabName, path);
            }
            if ((i % 4) == 3)
            {
                GUILayout.EndHorizontal();
                inHoriz = false;
            }
            i++;
        }
        if (inHoriz)
        {
            GUILayout.EndHorizontal();
        }
    }

    /*
     * Add prefab instance 
     */
    private void AddPrefabInstance(string name, string path)
    {
        foreach (GameObject obj in Selection.gameObjects)
        {
            PlayableDirector director = obj.GetComponent<PlayableDirector>();
            if (director != null)
            {
                var child = obj.transform.Find(name);
                if (child == null)
                {
                    // Does not already exist, lets try and add it.
                    var prefab = (GameObject)AssetDatabase.LoadAssetAtPath(path, typeof(GameObject));
                    if (prefab == null)
                    {
                        Debug.LogError("Could not load '" + name + "' at path '" + path + "'.");
                    }
                    else
                    {
                        // Add prefab game object as child.
                        var inst = PrefabUtility.InstantiatePrefab(prefab) as GameObject;
                        Undo.RecordObject(inst.transform, "Add " + name + " to sequence");
                        inst.transform.parent = obj.transform;

                        // If we find the Timeline, add an animation track to the timeline as well.
                        TimelineAsset timeline = (TimelineAsset) director.playableAsset;
                        if (timeline != null)
                        {
                            AnimationTrack track = timeline.CreateTrack<AnimationTrack>(null, name);
                            director.SetGenericBinding(track, inst); 
                        }
                    }
                }
            }
        }
    }
}